1. Field of the Invention
The present invention relates to a positive-displacement dry pump.
2. Background Art
For vacuuming, a dry pump has been used. The dry pump is provided with a pump chamber in which a rotor is contained in a cylinder. In the dry pump, a rotor rotates in a cylinder, an exhaust gas is compressed and moves, and vacuuming is performed so as to reduce the pressure of a sealed space provided at an intake (for example, refer to Published Japanese Translation No. 2004-506140 of PCT International Publication).
Specifically, in a case where vacuuming is performed so as to obtain a medium vacuum or an excellent vacuum, a multiple-stage dry pump is used in which a plurality of pump chambers are connected in series from the intake of the exhaust gas to a discharge port (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2003-166483).
When a dry pump is driven, an exhaust gas is compressed in the pump chamber and a heat is generated, and the temperature of a cylinder thereby rises.
When a temperature of the cylinder rises, the vacuuming efficiency is degraded.
Consequently, a dry pump is conventionally known, having a cooling medium path through which a cooling medium passes and which is formed at a peripheral portion of the cylinder, and uniformly cools down the entirety of the cylinder.
However, in the structure of the multiple-stage dry pump, as the pump chamber approaches an air side (discharge side), the inner pressure thereof may rise.
Because of this, as the pump chamber approaches an air side (discharge side), the amount of heat generation thereof may increase.
In the structure uniformly cooling down the entirety of the cylinder using a cooling medium or the like such as a conventional structure, difference in temperature between pump chambers occurs, and it is impossible to maintain the entirety of the dry pump at a uniform temperature. If a biased temperature occurs inside a dry pump, the dry pump is locally deformed, expanded, or the like, there is a problems in that the vacuuming efficiency is degraded.